Researchers say many mysterious new diseases are attacking coral reefs around the world, and that corals, some centuries old, are quickly succumbing; rash of new diseases takes researchers by surprise; fewer than dozen of new diseases even have names, and simply recognizing whether coral is healthy or sick can be extremely difficult, even for practiced researchers; scientists say multiple stresses, like bleaching, sedimentation and pollution, may have pushed corals to point where they cannot fend off diseases they have fought off in past; others suggest that dumping of sewage into sea has brought host of new pathogens in contact with corals; fact that basic information about diseases is unknown fills researchers with dread about future of corals around world.